In the past either telephone lines or subsidiary communication authorization (SCA) systems which involve subcarriers on FM broadcast stations were utilized to transmit background music and the like. Land line systems are expensive, while SCA systems which broadcast on subcarriers of FM radio stations in the 88-108 MHz band, are prone to noise and only facilitate two good quality subcarriers. Moreover the bandwidth of both systems is only 5 KHz which eliminates most high frequency audio components.
On the other hand, multi-point distribution system (MDS) channels operating at 2 GHz have in the past been utilized for dissemination of video to a limited number of locations. Originally, the MDS common carrier system was authorized for only point to point video applications. Because MDS systems were used exclusively for video programs, this particular service was underutilized and the Federal Communications Commission has now provided licenses for audio programs to be transmitted via microwave as a replacement for sub-carrier authorization service or the use of telephone lines.
In order to adapt the video MDS system to the provision of multiple audio programs, in the past it has been suggested that one transmit audio program material on the microwave TV audio channel, with sub-carriers multiplexed to provide for multiple program channels. This permits a 200 KHz audio frequency response range so as to accommodate and surpass the requirements of high fidelity material. The problem with this system is that since there is no video transmitted there is an exorbitant amount of wasted energy transmitted. This is because video related signalling such as the video carrier and color bursts are transmitted even if there is no video. This means that the effective power of the audio channel is reduced dramatically. For instance, assuming 100 total watts power, the entire audio transmission can only utilize approximately 25 percent of the allocated power. This dramatically reduces the possible coverage to a quarter of what it could have been had all of the energy been concentrated in audio programming. Note, with respect to MDS systems the transmission is from a single transmitter location to multiple points which gives rise to the designation of multiple-point distribution system.